1. Field of the Invention
The present invention relates to a novel acid anhydride ester and a composition of the ester, and a novel heat-curable resin composition and a cured product of the composition, and more specifically, to an acid anhydride ester and a composition of the ester, and a heat-curable resin composition and a cured product of the composition each of which is useful as: an encapsulant for a photoelectric conversion element typified by a light emitting diode such as a blue or white light emitting diode (hereinafter, abbreviated as “LED”); a raw material for a coating, adhesive, molded article, curing agent, polyimide resin, or the like; a modifier; a plasticizer; a lubricating oil raw material; an intermediate for a drug or agricultural chemical; a raw material for a coating; a toner resin; or the like.
2. Description of the Related Art
In recent years, blue LED's and white LED's each having high luminance have been developed, and have been finding use in an expanded variety of applications including bulletin boards, full-color displays, and backlights for portable phones. An acid anhydride curable epoxy resin has been conventionally used as an encapsulant for a photoelectric conversion element such as an LED because the resin is excellent in colorless and transparent nature. An alicyclic acid anhydride such as methyltetrahydrophthalic anhydride, hexahydrophthalic anhydride, or tetrahydrophthalic anhydride has been generally used as a curing agent for an epoxy resin for use in such photoelectric conversion element. Of those, methylhexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, or the like which is a liquid at normal temperature has been mainly used because of its ease of handling.
However, when the above alicyclic acid anhydride is used as a curing agent, a curing accelerator must be added to the alicyclic acid anhydride in order that the alicyclic acid anhydride may be sufficiently cured because the alicyclic acid anhydride has low curing reactivity.
For example, triphenylphosphonium bromide (JP2000-344868 A), 2-ethyl-4-methylimidazole (JP 2001-114868 A), an ethyl hexanoate of 1,8-diazabicyclo[5.4.0]undecene-7 (JP2002-97251 A: Example 8), and tetraphenylphosphonium bromide (JP 2003-26763 A) have been used as curing accelerators each used in a curing agent made of an alicyclic acid anhydride.
An epoxy resin composition as an encapsulant for a photoelectric conversion element is exposed to the strong light emission energy of an LED at a high temperature for a long time period. Accordingly, the epoxy resin composition must show a colorless and transparent nature under heating for a long time period in order that the epoxy resin composition may find applications in blue LED's and white LED's. However, such curing accelerator as described above is not preferably used in an encapsulant for a photoelectric conversion element such as an LED because of the following reason: when the curing accelerator is used in a curing agent made of an alicyclic acid anhydride, the curing accelerator itself changes to a yellow color owing to heating for a long time period, with the result that the colorless and transparent nature of the encapsulant is impaired.
In addition, a conventional curing agent has a high vapor pressure, and part of the agent evaporates at the time of curing. Accordingly, the compounding ratio of the agent deviates from a target value, and it becomes difficult to obtain a cured product having target performance.
There has been proposed an epoxy resin composition using cyclohexane-1,2,4-tricarboxylic acid-1,2-anhydride as a curing agent (JP 2005-36218 A and WO 2005/49597). The composition has excellent curability without the use of any curing accelerator. A cured product of the composition is colorless and transparent, and changes its color to a small extent even when the cured product is irradiated with light having a high temperature and high energy. The composition can find applications in blue LED's and white LED's. However, the curing agent is a solid or a liquid having an extremely high viscosity at room temperature, so the ease with which the curing agent and any other component of the composition are blended with each other is problematic.